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- Is Earth a "lucky" planet,
dwelling in a rare gem of a solar system that somehow escaped the gravitational
wrecking balls that have knocked other planetary systems cockeyed? Or is
the sun's realm fairly typical in its symmetry and orderliness?
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- Such questions arise from the extraordinary
burst of discovery that began in 1995, in which astronomers say they have
found 17 worlds orbiting sun-like stars outside the solar system -- and
counting.
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- The tally is getting high enough to show
statistical patterns that, scientists say, may be telling them something
new about how nature forms and destroys planets, and just how fragile the
prospects for life-harboring worlds like Earth may be. The growing population
of known extrasolar planets so far offers shocking contrasts with our own
home solar system, defying theories based on that familiar model and challenging
scientists to cobble together new ones.
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- All 17 extrasolar bodies appear to be
roughly as massive as the largest planet in our solar system -- the giant
gas ball Jupiter. It is not surprising that the hunters would find the
largest, most obvious planets first. But eight of them are bound in tight,
circular orbits that skim astoundingly close to their stars. The closest
of these "hot Jupiters" whips around its star at such scorching
proximity that its "year" -- one complete circuit -- lasts just
3.3 days.
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- Physical laws seem to dictate that they
could not have formed that close and so must have migrated inward early
in the system's history (wreaking havoc on any other fledgling worlds in
their path). And yet something has halted their gravitational death spiral
before it carried them "down the drain" to destruction.
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- All of the remaining nine extrasolar
planets -- those maintaining average distances of at least 19 million miles
from their stars (about one-fifth Earth's distance from the sun) -- follow
"eccentric" (egg-shaped) orbits -- killer orbits.
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- "A trend is now being stamped on
these discoveries that we thought, frankly, would go away," said Geoffrey
Marcy of San Francisco State University at a recent meeting of the American
Astronomical Society in Austin. "We are realizing that most of the
Jupiter-like planets . . . tool around in elliptical orbits, not circular
orbits, which are the rule in our solar system."
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- This seemingly minor distinction could
spell the difference between life and its absence. Jupiter-size bodies
lunging close and then veering far away from their stars are likely to
sweep away smaller worlds. "None of us would be here if we had such
a solar system," he said. "These Jupiters constitute a death
knell for any possible habitable planet as we know it."
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- That's "bad news" for those
in search of extraterrestrial intelligence, said Marcy, whose team leads
the world in planet detection. But he noted that, of hundreds of stars
surveyed so far, only about 5 percent appear to have world-wrecking planets.
"The good news is that 95 percent of the sun-like stars don't have
these nemeses."
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- Like the first photographs of Earth taken
from the moon, the revelations of the extrasolar worlds may inspire a new
appreciation for the fragility of human existence. Pointing to the bull's-eye
symmetry of the inner solar system, Marcy said, "Look at how perfect
this thing is. It's like a jewel. You've got circular orbits. They're all
in the same plane. They're all going around in the same direction. . .
. It's perfect, you know. It's gorgeous. It's almost uncanny."
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- Passing "Jupiters" flung out
of other systems, black holes and neutron stars that are known to wander
the galaxy are among the marauders that might have come barreling through
in the 4.6-billion-year history of the solar system, he said. "Obviously
our solar system represents kind of a dartboard. And no darts have hit
it."
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- While most astronomers have been convinced
that the extrasolar objects are indeed planets, broadly defined, a few
skeptics have pointed to the non-circular orbits as evidence to the contrary.
"It was the black-sheep property of what we've been discovering. It
didn't smell like the planets in our solar system," Marcy said. But
the claim that all planetary orbits must be like ours, he quipped, "may
well be a circular argument."
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- The discoveries have pushed the very
definition of the term "planet" into a state of chaos that dwarfs
the periodic flaps over Pluto. The conventional definition, reflecting
our own solar system, holds that planets form from a flat disk of dust
and gas rotating around a newly forming star. Within this framework, planets
might vary enormously, from small and rocky (like Earth and its near neighbors)
to unsurfaced gas giants -- and tiny, icy eccentrics such as Pluto.
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- Theorists have found ways to explain
some, but not all, of the properties of the newfound planets, said astronomer
William Cochran of the University of Texas. In any case, even if the objects
don't quite fit the mold, he added, "right now, we are still going
to call them 'planets.' "
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- Theorists have calculated how planets
born in the expected circular orbits might get kicked onto different paths
in gravitational encounters with other objects (rival planets, passing
stars, etc.). For example, the nine extrasolar eccentrics "may be
in systems that started more or less like our own, with one Jupiter,"
Marcy said, "but maybe the Saturn grew too big or too close. And those
two gravitationally slingshot off of each other," one being catapulted
into interstellar space, the other spiraling in toward its star.
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- To discover how rare the solar system
really is will require more time and data. No technology has yet proven
capable of detecting Earth-size planets, and even the big ones are hidden
from view in the overpowering glare of starlight. The leading detection
technique is a complicated, indirect one that measures the very slight
wobble induced in a star by the gravity of its unseen planet.
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- This and other techniques should gradually
reveal lower-mass planets and planets of all sizes orbiting at greater
distances from their stars. So far, no more than one planet has been confirmed
around any sun-like star, but some of the data hint at what could be multi-planet
families.
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- "We'll probably find many more surprises,
strange systems that will make us rethink what we're doing. But we should
also find lots of systems that look something like our solar system,"
said theorist Alan P. Boss of the Carnegie Institution of Washington.
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- Dozens of teams are working on improved
methods and tools. Studies of dust disks around nearby stars -- the potential
breeding grounds of planets -- have begun to reveal details about the processes
within. And two international groups have reported the first results from
a technique called microlensing, which might be used to detect planets
as small as Earth.
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- In little more than a decade, astronomers
expect to complete the first census of planets orbiting nearby stars. And
NASA plans to deploy a series of space-borne telescopes that could lead
to a mighty Planet Finder capable of producing actual images of distant
Earth-like planets. Assuming a killer Jupiter didn't find them first.
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